1. Field of the Invention
The present invention relates to a packet control system, which divides pieces of information into predetermined bit strings and transmits them, and more particularly to a packet control system which interleaves a packet, which includes information requiring urgency, between other packets.
2. Description of the Related Art
In well known packet communications, information transmitted between a transmission side and a receiving side has been heretofore stored in a data string of a finite length as shown in FIG. 1, which is called a packet. To realize this packet communication in physical layer, conditions must be defined so that a packet transmitted from the transmission side can be read correctly by the receiving side. The most elementary conditions are to define a correspondence between electrical, magnetic and optical waveforms being transmitted through a communication medium and a bit (generally, 0 or 1) that is a unit of information. However, such conditions are not satisfactory enough to establish the packet communication.
Specifically, the following items must be decided.
a) a method for displaying a header and tail of a packet and for reading these headers and tails
b) correspondence between positions of bits (1, 0) existing in the packet and transmission information (so called a packet format).
As the foregoing conditions for displaying the header of the packet, there are the following methods.
1) a method for defining a specific bit string that can be uniquely distinguished
2) a method for deciding as a starting point of the packet a time point when a specific bit string first appears after a later-described idle.
Furthermore, as an condition for displaying the tail of the packet, there are the following methods.
3) a method of defining a specific bit string that can be uniquely distinguished.
4) a method for displaying a length of a packet in a packet format.
5) a method for defining a bit string before an idle as a packet body. Note that a specific bit string displaying a header of a packet shall be called “a start-of-packet delimiter” and a specific bit string displaying a tail of the packet shall be called “an end-of-packet delimiter”.
The information to be transmitted in the packet communication does not continue to be generated temporally, but an idle state where no information exists is present in the packet communication. As a condition for distinguishing this idle state, there are the following methods.
6) a method for defining a specific bit string that can be uniquely distinguished.
7) a method for creating a non-signal state (state where no electrical, magnetic and optical waveforms are present).
When the conventional packet communication is illustrated schematically based on the above descriptions, the conventional packet communication is shown in FIG. 2. In FIG. 2, a start-of-packet delimiter is added to the header of a packet, and an end-of-packet delimiter is added to the rear portion thereof.